The invention applies advantageously but not exclusively to the field of motor vehicles for the protection of an electronic entity such as, for example, a control unit or a processor, against a reversal of the expected polarity of the DC power supply voltage intended for this electronic entity.
The devices for protecting against polarity reversals of a DC power supply voltage make it possible to avoid the flow of reverse currents through electronic components during said polarity reversal.
Currently, there are a number of solutions for producing protection against these polarity reversals. Notable among these is the series connection of diodes in the power supply line. However, such a solution generates unacceptable power dissipations when the current in the electronic entity to be protected, a control unit for example, exceeds 5 amps. Now, electronic control units that can be used in motor vehicle applications can control charging currents ranging from 20 to 40 amps. Furthermore, the series-connected diodes in conjunction with the inductance of the power supply line, and the capacitors of the power supply filters usually present in the electronic control units, create a rectified resonant circuit which can generate overvoltage conditions on the power supply line of the controlling electronic unit to be protected.
Another solution consists in inserting an electromechanical relay into the positive power supply line, the command coil being powered between the positive and negative power supply terminals. Such an arrangement forces the relay to close in the presence of a minimum voltage difference between the positive and negative terminals of the power supply and forces the relay to open if the voltage present at the positive power supply terminal drops to a value close to zero or becomes negative relative to the negative power supply terminal.
Such a solution does not present the drawbacks of the previous solution, but does require bulky relays, which may be incompatible with the physical place that is to receive the electronic entity to be protected. Furthermore, such relays must be assembled manually on the electronic entity when the current conveyed by the latter exceeds a few amps, because such relays cannot be mounted as surface components.
Another solution consists in placing a P-channel MOS transistor in the positive power supply line. Although this solution offers the benefit of the simplicity of a command circuit for the PMOS transistor, it nevertheless presents a drawback in terms of surface footprint because of the large size of a power PMOS transistor.
Another solution consists in placing an N-channel MOS transistor in the negative polarity line. However, such a solution requires the negative output terminal of the electronic unit to be protected, to which loads are connected, to be connected in return to the negative power supply terminal of this electronic unit so as to avoid the flow of negative currents in all the loads connected to the electronic unit during a polarity reversal of the power supply voltage. Now, in some applications, for example motor vehicle applications, the negative terminals of the loads are directly connected in return to the negative power supply terminal without going back through the electronic unit which controls these loads.
Furthermore, analogue components external to the electronic unit, sensors for example, may not be electrically compatible with electronic unit input terminals becoming negative during a polarity reversal.
It has also been proposed in the prior art to insert an N-channel transistor into the positive power supply line, associated with an external discrete command component, such as a charging pump, capable of supplying a command voltage to the gate of the transistor that is greater than the power supply voltage in normal operation. However, such a solution using an external discrete charging pump has a significant surface footprint, and causes a strong electromagnetic noise to be generated because of the relatively high value of the capacitors of the charging pump used in discrete charging pump implementations.